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Date:   Fri, 16 Aug 2019 08:20:36 -0400
From:   "M. Vefa Bicakci" <m.v.b@runbox.com>
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Platform Driver <platform-driver-x86@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@intel.com>,
        Vishwanath Somayaji <vishwanath.somayaji@intel.com>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>
Subject: Re: [PATCH] platform/x86: intel_pmc_core: Do not ioremap RAM
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On Fri, 16 Aug 2019 12:21:58 +0300
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Fri, Aug 16, 2019 at 4:42 AM M. Vefa Bicakci <m.v.b@runbox.com> wrote:
> >
> > On a Xen-based PVH virtual machine with more than 4 GiB of RAM,
> > intel_pmc_core fails initialization with the following warning message
> > from the kernel, indicating that the driver is attempting to ioremap
> > RAM:
> >
> >   ------------[ cut here ]------------
> >   ioremap on RAM at 0x00000000fe000000 - 0x00000000fe001fff  
> 
> > This issue appears to manifest itself because of the following fallback
> > mechanism in the driver:
> >
> >         if (lpit_read_residency_count_address(&slp_s0_addr))
> >                 pmcdev->base_addr = PMC_BASE_ADDR_DEFAULT;
> >
> > The validity of address PMC_BASE_ADDR_DEFAULT (i.e., 0xFE000000) is not
> > verified by the driver, which is what this patch introduces. With this
> > patch, if address PMC_BASE_ADDR_DEFAULT is in RAM, then the driver will
> > not attempt to ioremap the aforementioned address.  
> 
> Thank you for the patch.

Hello Andy,

Thank you for reviewing the patch!

> Is there anything preventing us to use memremap() in such case?

I re-read the documentation for memremap a few times along with taking
a look at its code, but I think I am missing an important piece of
information. As I understand it, depending on its flags, memremap would
allow a section of RAM to be mapped for the PMC driver.

The intention with this patch is to prevent the driver from being
instantiated when the default/fallback memory address is in RAM, as
this issue occurs with a non-administrative virtual machine (domU in
Xen terminology) that does not simulate or pass-through a corresponding
PMC device.

I think that I have misunderstood your review comment though, so I
would apppreciate it if you could elaborate.

Thanks again for reviewing the patch,

Vefa

(Please note that my next reply may be delayed by about 10 hours.)